That is a standard practice in the 303 Brits. It keeps the case centered in the large military chamber so it expands concentric. The secret is the to neck size only the best way with the Lee collet sizing die. [Lee is the best for some things , for the Lee haters]

Using the collet die keeps the shoulder to where it had expanded on the first shot. The O ring kept the brass round the size of the chamber.

I get 10 to 15 shots from a case this way.